1. Field of the Invention
The present invention relates to a glass preform for use in the fabrication of a dispersion shifted single mode optical fiber and a method for the production of said glass preform. More particularly, it relates to a glass preform for use in the fabrication of a dispersion shifted single mode optical fiber (hereinafter referred to as "dispersion shifted optical fiber") which has a zero dispersion wavelength in a 1.5 .mu.m wavelength band and a method for the production of such glass preform.
2. Description of the Prior Arts
A quartz base optical fiber has minimum attenuation of light transmission in a 1.5 .mu.m wavelength band (1.50-1.60 .mu.m), and a dispersion shifted optical fiber which has a zero dispersion wavelength in the 1.5 .mu.m wavelength band has been studied and developed for use as a long distance optical communication line with a large transmission capacity.
Among the dispersion shifted optical fiber, one having a convex refractive index profile shown in FIG. 1 has less flexural loss than other dispersion shifted optical fibers having simple step-like refractive index profile and better practical advantages, and intensively developed (cf. Ohashi et al, "Characteristics of a dispersion-shifted fibers with a convex profile", National Conference Record, 1985, The Institute of Electronics and Communication Engineers of Japan, The Section of Semiconductor Devices and Materials, paper 413; N. Kuwaki, et al, "Dispersion-shifted convex-index single mode fibers", Electronics Letters, Vol. 21, No. 25/26, 1186-1187, Dec. 5, 1985); and N. Kuwaki et al, "Characteristics of distribution-shifted convex-index fibers with graded center-core", National Conference Record, 1986, The Institute of Electronics and Communication Engineers of Japan, paper 1072).
The refractive index profile of FIG. 1 consists of the largest refractive index 1 corresponding to an inner core, a refractive index 2 smaller than the highest refractive index 1 corresponding to an outer core which surrounds the inner core and the smallest refractive index 3 corresponding to a cladding which surrounds the outer core.
In a dispersion shifted optical fiber having a convex refractive index profile, the refractive index of the inner cor is larger than that of the cladding by about 1.0%. To achieve such refractive index difference between the core and the cladding, GeO.sub.2 is generally added to quartz glass of the core to increase its refractive index. However, when the refractive index of the core is increased only by the addition of GeO.sub.2, Rayleigh scattering in the glass increases and in turn the attenuation of light transmission of the optical fiber increases. Further, electron transition absorption in UV light range due to reduction of GeO.sub.2 to GeO increases, and its influence reaches to the 1.5 .mu.m wavelength band which is used for light transmission, whereby the attenuation of light transmission increases.
Another way to achieve such refractive index difference between the core and the cladding, B.sub.2 O.sub.3 or fluorine is added to the cladding glass to decrease its refractive index. Particularly, fluorine is useful to produce the optical fiber having low attenuation since it does not have any absorption band near the 1.5 .mu.m wavelength band while B.sub.2 O.sub.3 has such absorption band. Therefore, the decrease of the added amount of GeO.sub.2 to the core by the addition of fluorine to the cladding is an effective measure to decrease attenuation of light transmission of the optical fiber. As the dispersion shifted optical fiber having the convex refractive index profile and containing fluorine in the cladding, proposed was an optical fiber having glass compositions as shown in FIG. 2, which comprises an inner core 21 made of GeO.sub.2 -SiO.sub.2 glass, an outer core 22 made of SiO.sub.2 glass and a cladding 23 made of F-SiO.sub.2 glass (cf. H. Yokota et al, "Dispersion-shifted fibers with fluorine added cladding by the vapor phase axial deposition method", Technical Digest on Topical Meeting on Optical Fiber Communication (Atlanta, 1986), Paper WF2).
For mass production of a glass preform for an optical fiber, the VAD (Vapor Phase Axial Deposition) method is known and widely employed. However, it is very difficult to produce a glass preform having a complicated refractive index profile suitable for the dispersion shifted optical fiber in which GeO.sub.2 and fluorine are selectively added to the inner core and the cladding, respectively by the VAD method.
In addition, according to the above proposal by Yokota et al, the attenuation of light transmission is reduced by the addition of fluorine to the cladding so as to decrease the amount of GeO.sub.2 added only to the inner core. According to this proposal, it is possible to decrease attenuation of light transmission of the dispersion shifted optical fiber at a wavelength of 1.55 .mu.m as reported by Shigematsu et al ("Transmission Characteristics of Dispersion-shifted Single-mode Fibers", Technical Study Reports, The Institute of the Electronics and Communication Engineers of Japan, OQE 86-99). In this report, the dispersion shifted optical fiber had a refractive index profile as shown in FIG. 2 and comprised an inner core of 3 .mu.m in diameter (a), an outer core of 9 .mu.m in outer diameter (b) and a cladding of 125 .mu.m in outer diameter (c).
However, it is very difficult and almost impossible to further decrease attenuation of light transmission of the dispersion shifted optical fiber comprising the inner core made of GeO.sub.2 -SiO.sub.2 glass, the outer core made of SiO.sub.2 glass and the cladding made of F-SiO.sub.2 glass in the 1.5 .mu.m wavelength band.